Allow specifying tags in `DoNotMirror` field

[adamkpickering]

Right now, the DoNotMirror field takes only a bool. This is problematic for images where we want to mirror only some tags (for example, the registry.suse.com/rancher/elemental-operator image). The current solution is to make two entries for the same SourceImage-TargetImageName pair in Images, but this is suboptimal. This issue is about fixing this.

Acceptance Criteria

• The DoNotMirror field can take either a bool (which causes all tags to not be mirrored) or an array of strings.
• The changes to the DoNotMirror field are documented.
• config.yaml and regsync.yaml are updated to exploit the changes.